The present invention relates to an engine intake apparatus wherein the valve timing between intake and exhaust valves is variable in accordance with an engine speed.
There has been conventionally proposed various engine intake systems equipped with a variable valve timing mechanism capable of varying the valve timing between intake and exhaust valves. The mutual relationship between intake and exhaust valves is normally defined or represented in terms of a valve overlap. The valve overlap is generally defined by a specific crank angle during which the intake and exhaust valves open simultaneously. In an engine transition period from the exhaust stroke to the intake stroke, the exhaust valve normally comes to close while the intake valve comes to open. In this transition period, both the intake and exhaust valves are opened simultaneously during a predetermined crank angle. This period is referred to as a valve overlap.
The valve overlap is generally controlled to be large or small to optimize various engine performances such as scavenging efficiency, charging efficiency, combustion stability in accordance with an engine operational condition.
For example, in an engine equipped with a supercharger disclosed in Unexamined Japanese Patent Publication No. 2-119641, there is disclosed a typical variable valve timing mechanism. More specifically, a valve overlap is usually widened in an engine high load condition, so that residual exhaust gas can be sufficiently scavenged by high pressure of supercharged intake air during this relatively long overlap period. On the other hand, the valve overlap is narrowed in an engine low load condition, so that stability of combustion is ensured.
It is also known that widening the valve overlap in accordance with am increase of the engine speed is effective to more precisely optimize the scavenging performance.
Meanwhile, as a means for increasing intake air charge amount, there has been conventionally known a dynamic supercharging system utilizing inertia or resonance effect. This dynamic supercharging system basically varies an effective length of intake air passage in accordance with an engine speed so as to cause a dynamic supercharging effect in a wide engine speed range. If such a dynamic supercharging system is incorporated into an engine equipped with a mechanical supercharger, a load of the mechanical supercharger is fairly reduced and therefore an overall engine torque will be largely increased. It is needless to say that, even if this dynamic supercharging system is incorporated into a normal aspiration engine, an engine torque will be also increased.
This kind of dynamic supercharging system basically utilizes a pressure wave propagating in the intake air passage. More specially, in order to increase intake air charge amount, it is essential to supply a high positive-pressure wave to the engine cylinder at the terminal end of its intake stroke. In other words, the conventional dynamic supercharging system mainly pays attention to how a high positive-pressure is caused at the final stage of the intake stroke.
However, the inventors of this application found that the pressure condition during the valve overlap period significantly affects engine performance if such a dynamic supercharging system is united with the variable valve timing mechanism.
In case of inertia supercharging arrangement, a negative-pressure wave is caused due to suction motion of an engine piston during an intake stroke of an engine. This negative-pressure wave propagates in the intake air passage from the engine cylinder to an upstream part thereof. Then, this negative-pressure wave is reflected at an opened end (i.e. a volumetrically enlarged portion) of the intake air passage. The reflected wave becomes a positive-pressure wave and returns to the engine cylinder. Namely, the positive-pressure wave is finally supplied to the engine cylinder at the terminal stage of the intake stroke.
Concerning the pressure condition through this inertia supercharging behavior, there is caused a small pressure variation during the valve overlap period because substantially no large change is generated in the very beginning of the intake stroke, although the negative pressure is soon generated by the suction motion of the engine piston after this overlap period.
On the contrary, in case of resonance supercharging arrangement, a plurality of engine cylinders are divided into two groups so that engine cylinders belonging to the same group are not contiguous with each other in their intake orders. For this arrangement, cylinders belonging to the same group cooperate to cause a stable pressure oscillation in the intake air passage due to cyclically repeated intake motions of respective cylinders in the same group. In this case, a relatively large positive-pressure meets the valve overlap period of each cylinder because of a resonance wave stably residing in the intake air passage. Accordingly, the intake air pressure condition is very different between the inertial and resonance supercharging conditions.
Hence, it will be a key point in further improving the engine performance to manage both the length of the valve overlap period and the intake air pressure condition during the valve overlap period.